1. Field of the Invention
The present invention relates to a fabrication method of a microstructure array, a fabrication method of a mold or a master of a mold (in the specification the term xe2x80x9cmoldxe2x80x9d is chiefly used in a broad sense including both a mold and a master of a mold) for forming a microstructure array, a fabrication method of a microstructure array using the mold, and a microstructure array. This invention particularly relates to a mold for forming a microlens array, a fabrication method of the mold, and a fabrication method of the microlens array using the mold.
2. Description of the Related Background Art
A microlens array typically has a structure of arrayed minute lenses each having a diameter from about 2 or 3 microns to about 200 or 300 microns and an approximately semispherical profile. The microlens array is usable in a variety of applications, such as liquid-crystal display devices, optical receivers and inter-fiber connections in optical communication systems.
Meanwhile, earnest developments have been made with respect to a surface emitting laser and the like which can be readily arranged in an array with narrow pitches between the devices. Accordingly, there exists a significant need for a microlens array with narrow lens intervals and a large numerical aperture (NA).
Likewise, a light receiving device, such as a charge coupled device (CCD), has been more and more downsized as semiconductor processing techniques develop and advance. Therefore, also in this field, the need for a microlens array with narrow lens intervals and a large NA is increasing.
In the field of such a microlens, a desirable structure is a microlens with a large light-condensing efficiency which can highly efficiently utilize light incident on its lens surface.
Further, similar desires exist in prospective fields of optical information processing, such as optical parallel processing-operations and optical interconnections. Furthermore, display devices of active or self-radiating types, such as electroluminescence (EL) panels, have been enthusiastically studied and developed, and a highly-defined and highly-luminous display has been thus proposed. In such a display, there is a heightened desire for a microlens array which can be produced at a relatively low cost and with a large area as well as with a small lens size and a large NA.
There are presently a number of prior art methods for fabricating microlenses.
In a prior art microlens-array fabrication method using an ion exchange method (see M. Oikawa, et al., Jpn. J. Appl. Phys. 20(1) L51-54, 1981), a refractive index is raised at plural places in a substrate of multi-component glass by using an ion exchange method. A plurality of lenses are thus formed at high-refractive index places. In this method, however, the lens diameter cannot be large, compared with intervals between lenses. Hence, it is difficult to design a lens with a large NA. Further, the fabrication of a large-area microlens array is not easy since a large scale manufacturing apparatus, such as an ion diffusion apparatus, is required to produce such a microlens array. Moreover, an ion exchange process is needed for each glass, in contrast with a molding method using a mold. Therefore, variations of lens quality, such as focal lengths, are likely to increase between lots unless the management of fabrication conditions in the manufacturing apparatus is carefully conducted. In addition to the above, the cost of this method is relatively high, as compared with the method using a mold.
Further, in the ion exchange method, alkaline ions for ion-exchange are indispensable in a glass substrate, and therefore, the material of the substrate is limited to alkaline glass. The alkaline glass is, however, unfit for a semiconductor-based device which needs to be free of alkaline ions. Furthermore, since a thermal expansion coefficient of the glass substrate greatly differs from that of a substrate of a light radiating or receiving device, misalignment between the microlens array and the devices is likely to occur due to a misfit between their thermal expansion coefficients as an integration density of the devices increases.
Moreover, a compressive strain inherently remains on the glass surface which is processed by the ion exchange method. Accordingly, the glass tends to warp, and hence, a difficulty in joining or bonding between the glass and the light radiating or receiving device increases as the size of the microlens array increases.
In another prior art microlens-array fabrication method using a resist reflow (or melting) method (see D. Daly, et al., Proc. Microlens Arrays Teddington., p23-34, 1991), resin formed on a substrate is cylindrically patterned using a photolithography process and a microlens array is fabricated by heating and reflowing the resin. Lenses having various shapes can be fabricated at a low cost by this resist reflow method. Further, this method has no problems of thermal expansion coefficient, warp and so forth, in contrast with the ion exchange method.
In the resist reflow method, however, the profile of the microlens is strongly dependent on the thickness of resin, wetting condition between the substrate and resin, and heating temperature. Therefore, variations between lots are likely to occur while a fabrication reproducibility per a single substrate surface is high.
Further, when adjacent lenses are brought into contact with each other due to the reflow, a desired lens profile cannot be secured due to the surface tension. Accordingly, it is difficult to achieve a high light-condensing efficiency by bringing the adjacent lenses into contact and decreasing an unused area between the lenses. Furthermore, when a lens diameter from about 20 or 30 microns to about 200 or 300 microns is desired, the thickness of deposited resin must be large enough to obtain a spherical surface by the reflow. It is, however, difficult to uniformly and thickly deposit the resin material having desired optical characteristics (such as refractive index and optical transmissivity). Thus, it is difficult to produce a microlens with a large curvature and a relatively large diameter.
In another prior art method, an original plate of a microlens is fabricated, lens material is deposited on the original plate and the deposited lens material is then separated. The original plate or mold is fabricated by an electron-beam lithography method (see Japanese Patent Application Laid-Open No. 1 (1989)-261601), or a wet etching method (see Japanese Patent Application Laid-Open No. 5 (1993)-303009). In these methods, the microlens can be reproduced by molding, variations between lots are unlikely to occur, and the microlens can be fabricated at a low cost. Further, the problems of alignment error and warp due to the difference in the thermal expansion coefficient can be solved, in contrast with the ion exchange method.
In the electron-beam lithography method, however, an electron-beam lithographic apparatus is expensive and a large investment in equipment is needed. Further, it is difficult to fabricate a mold having a large area more than 100 cm2 (100 cm-square) because the electron beam impact area is limited.
Further, in the wet etching method, since an isotropic etching using a chemical action is principally employed, an etching of the metal plate into a desired profile cannot be achieved if composition and crystalline structure of the metal plate vary even slightly. In addition, etching will continue unless the plate is washed immediately after a desired shape is obtained. When a minute microlens is to be formed, a deviation of the shape from a desired one is possible due to an etching lasting during a period from the time a desired profile is reached to the time the microlens is reached.
Further, there also exists a mold fabrication method using an electroplating technique (see Japanese Patent Application Laid-Open No. 6 (1994)-27302). In this method, an insulating film having a conductive layer formed on one surface thereof and an opening is used, the electroplating is performed with the conductive layer acting as a cathode, and a protruding portion acting as a mother mold for a lens is formed on a surface of the insulating film. The process of fabricating the mold by this method is simple, and cost is reduced. Similar such methods are also disclosed in Japanese Patent Application Laid-Open No. 8 (1996)-258051 and Japanese Patent Publication No. 64 (1989)-10169.
The problem occurring when a plated layer is formed in an opening by the electroplating technique will be described by reference to FIGS. 1A and 1B. FIGS. 1A and 1B illustrate a radius variation or distribution of plated layers formed in a two-dimensional array on a substrate. In the above fabrication method using the electroplating in an electroplating bath, a distribution or variation of an electroplating-current density occurs over the substrate due to a pattern of the openings (i.e., the electrode pattern) when the electroplating is conducted. More specifically, the electric field is unevenly concentrated and the electroplating growth is hence promoted near the periphery of the pattern of the arrayed openings, as a result of which a distribution or variation of the size of semispherical microstructures 508 occurs in a usable region 505 of the substrate. Therefore, when this substrate is used as a mold for forming a microlens array, specifications of respective microlenses vary over the array.
An object of the present invention is to provide a method of fabricating a microstructure array (typically a microlens array such as a semispherical microlens array, a flyeye lens and a lenticular lens) flexibly, readily and stably performing, a fabrication method of a mold for forming a microstructure array, a fabrication method of a microstructure array using the mold, and so forth.
In a first aspect of the present invention, a fabrication method of fabricating an array of microstructures is provided. The method includes the step of preparing a substrate with a surface including a usable region and a dummy region continuously set around the usable region. At least the usable region and the dummy region of the substrate are electrically conductive and have a conductive portion. The method further includes the steps of forming a first insulating layer on the conductive portion, and forming a plurality of openings in the first insulating layer, the openings being arranged in a predetermined array pattern. Further, the method includes the step of performing one of electroplating and electrodeposition using the conductive portion as an electrode to form a first plated or electrodeposited layer in the openings and on the first insulating layer in both the usable region and the dummy region.
The predetermined array pattern is typically a two-dimensional array pattern which is periodical in at least one direction, a two-dimensional array pattern which is periodical in mutually-orthogonal four directions, or a periodical stripe pattern. In terms of an influence of the pattern on the distribution of a current density at the time of the electroplating or electrodeposition, the predetermined array pattern is a pattern that creates a current distribution in which the current density is substantially constant in its central portion and gradually increases toward its peripheral portion.
More specifically, the following constructions are possible based on the above fundamental construction.
The fabrication method may further include a step of forming a sacrificial layer under the conductive portion in the dummy region of the substrate, and a step of removing the sacrificial layer to remove the first plated or electrodeposited layer formed in the dummy region. In this case, the fabrication method may further include a step of forming a spacer layer under the conductive portion in the usable region of the substrate to equalize a height level of the conductive portion in the usable region with a height level of the conductive portion in the dummy region.
The fabrication method may further include a step of forming a step layer under the conductive portion in the dummy region of the substrate to differentiate height levels of the conductive portions between the usable region and the dummy region.
The usable region may contain a plurality of blocks. In this case, the blocks may be separated from each other by a boundary portion of the substrate. Further, in such a case, the fabrication method may include a step of forming a sacrificial layer under the conductive portion in the dummy region and the boundary portion of the substrate, and a step of removing the sacrificial layer to remove the first plated or electrodeposited layers formed in the dummy region and the boundary portion. In those methods, the blocks of the usable region may include the same array of the first plated or electrodeposited layers, or different arrays of the first plated or electrodeposited layers, respectively. Further, in those methods, the fabrication method may include a step of dividing the substrate into the blocks after the first plated or electrodeposited layers are formed.
The fabrication method may include a step of cutting off the dummy region of the substrate after the first plated or electrodeposited layers are formed. In this case, the fabrication method may include a step of bonding the substrate with only the usable region or the divided block to a supporting substrate.
The first electrodeposited layer of electrodepositable organic compound may be electrodeposited in the opening and on the first insulating layer, or the first plated layer may be electroplated in the opening and on the first insulating layer.
The fabrication method may further include a step of forming a second insulating layer on the usable region with the first plated layer, and a step of performing an electrolytic etching of the first plated layers in the dummy region without the second insulating layer by applying a voltage using the conductive portion as an anode. In this case, material of the first plated layer removed by the electrolytic etching can be collected in an electroplating bath or on an opposite metal electrode, and therefore, the material of the first plated layer can be reused without waste. Even when precious metal is used, the cost can be made relatively low.
The fabrication method may include a step of removing the second insulating layer after the electrolytic etching is performed, a step of entirely removing the first insulating layer, and a step of forming a second plated or electrodeposited layer continuously on the conductive portion in the dummy region and the first plated layer in the usable region. Since the second layer only needs to be continuously formed to firmly fix the first layer to the conductive portion, the second layer can be formed by electrodepostion or electroless plating as well as electroplating such as DC or pulsed electroplating. Dispersion electroplating, in which dispersion particles such as Al2O3, TiO2 and PTFE are added to the electroplating bath, can also be used. Mechanical strength and corrosion resistivity of the microstructure can be improved by the dispersion particles. As the electrodeposition substance electrodeposited on the first plated layer using a current, there can be employed an electrodepositable organic compound (acryl-series carboxylic acid resin and the like in the case of the anionic-type electrodeposition, and epoxy-series resin and the like in the case of the cationic-type electrodeposition).
The fabrication method may include a step of removing the first insulating layer not covered with the second insulating layer, and a step of removing the second insulating layer. In this case, the fabrication method may include a step of forming a second plated or electrodeposited layer continuously on the conductive portion in the dummy region and the first plated layer in the dummy region. Here, when the first insulating layer is composed of a material, such as phospho-silicate glass, which has a good adhesion property to the conductive portion, the substrate with only the first plated layer can be used as a mold.
The second plated layer may be formed by electroplating using the conductive portion and the first plated layer as a cathode, electrodeposition using the conductive portion and the first plated layer as an electrode, or electroless plating which can form the second plated layer with a high glossiness. In this case, the second plated layer may be composed of a nickel plated layer. The second plated layer of nickel may be composed of an electroless plated layer, or the second plated layer of nickel may contain phospher to improve its corrosion resistivity.
The fabrication method may include a step of forming an alignment marker composed of the first plated layer. The alignment marker may be composed of the first plated layer covered with the second insulating layer when the first plated layers in the usable region are covered with the second insulating layer.
The electrolytic etching may be performed using the same electroplating bath used when the first plated layer is formed.
The conductive portion acting as an electrode and the first plated layer may be composed of such materials that do not produce an alloy layer therebetween, respectively. Thereby, the first plated layers in an unneeded region (i.e., dummy region) can be completely removed without leaving any protrusions, and an undesired protrusion will not be formed in the dummy region when the second plated or electrodeposited layer is formed. Such an undesired protrusion is likely to be mistaken as an alignment marker, for example. The same effect can be achieved when the conductive portion and the first plated layer are composed of such materials that do not produce an alloy layer in the first plated layer due to a diffusion of the material of the conductive portion into the material of the first plated layer, respectively. For example, when the first plated layer is composed of Ni and the conductive portion is composed of Au, Au will be diffused into the first plated layer to produce an alloy layer in the first plated layer. In such a case, a protrusion of the alloy layer will be left when the first plated layer is removed.
In contrast, the conductive portion and the first plated layer may be composed of such materials that produce an alloy layer in the conductive portion due to a diffusion of the material of the first plated layer into the material of the conductive portion, respectively. In such a case, material of the first plated layer will be diffused into the conductive portion to produce an alloy layer in the conductive portion. Therefore, even if the alloy layer is left when the first plated layer is removed, the alloy layer will not create a protrusion in the dummy region. For example, when the first plated layer is composed of Au and the conductive portion is composed of Ni, Au will be diffused into the conductive portion to produce an alloy layer in the conductive portion which has no adverse influence.
A width of the dummy region of the substrate is preferably set to 2 mm or more. This value is independent of an area of the usable region. Such a value is needed to sufficiently unify the distribution of the electric field for electroplating or electrodeposition in the usable region and to push off a relatively large distribution portion into the dummy region. Thereby, the size distribution of the first plated or electrodeposited layers in the usable region can be readily and stably made 5% or less. Such a value makes it possible to achieve a microlens array and the like with a good performance. In this specification, the size or radius distribution is used as a ratio of a difference between a maximum value and an average value relative to the average value concerning the size or radius of first plated or electrodeposited layers in a certain region.
The opening may have a circular shape to obtain a semispherical microstructure, or may have an elongated stripe shape to obtain a semicylindrical microstructure.
The fabrication method may further include a step of forming a mold on the usable region of the substrate with the first plated or electrodeposited layer, and a step of separating the mold from the substrate. In this case, the mold may be formed using electroplating. The mold can be a mold for fabricating a microlens array.
The fabrication method may further include a step of coating a light-transmitting material on the mold, a step of hardening the light-transmitting material, and a step of separating the material from the mold to obtain the microlens array.
The fabrication method may further include a step of coating a light-transmitting material on another substrate, a step of pressing the mold against the light-transmitting material on the substrate, a step of hardening the light-transmitting material, and a step of separating the material from the mold to obtain the microlens array.
The first plated layers may be formed separately from each other. In this case, the first plated layers in the dummy region not covered with the second insulating layer can be electrolytically etched accurately. Further, the first insulating layer can be entirely removed accurately when it is desired.
The fabrication method may further include a step of performing electroplating on the usable region of the substrate to smooth a protruding portion of the first insulating layer at a periphery of the usable region for the purpose of easily handling the substrate.
In another aspect of the present invention, a microstructure array is provided. The microstructure array includes a substrate with a surface including a usable region and a dummy region continuously set around the usable region, at least the usable region and the dummy region of the substrate are electrically conductive and have a conductive portion. The microstructure array also includes a first insulating layer formed on the conductive portion and a plurality of openings formed in the first insulating layer. The openings are arranged in a periodical array pattern. A first plated or electrodeposited layer is formed as a microstructure in the openings and on the first insulating layer in each of the usable region and the dummy region.
In yet another aspect of the present invention, a microstructure array is provided. The microstructure array includes a substrate with a surface including a usable region, the usable region of the substrate being electrically conductive and having a conductive portion. The microstructure array also includes a first insulating layer formed on the conductive portion, and a plurality of openings formed in the first insulating layer. The openings are arranged in a periodical array pattern. A first plated or electrodeposited layer is formed as a microstructure in the openings and on the first insulating layer, and a distribution of a radius of the microstructure is within 5%. Further, a supporting substrate is provided, the substrate being bonded to the supporting substrate
In another aspect of the present invention, a microstructure array is provided. The microstructure array includes a substrate with a surface including a usable region and a flat region continuously set around the usable region. The usable region of the substrate is electrically conductive and has a conductive portion. A first insulating layer is formed on the conductive portion, and a plurality of openings are formed in the first insulating layer, the openings being arranged in a periodical array pattern. A first plated or electrodeposited layer is formed as a microstructure in the openings and on the first insulating layer, and a radius distribution of the microstructure is within 5%.
In another aspect of the present invention, a mold for fabricating a convex microlens array is provided. The mold includes a plurality of openings formed in a first insulating layer, the openings being arranged in a periodical array pattern. The mold also includes a first plated or electrodeposited layer formed as a microstructure in the openings and on the first insulating layer. A radius distribution of the microstructure is within 5%.
In yet another aspect of the present invention, a microstructure array is provided. The microstructure array includes a substrate with a surface including a usable region and a flat region continuously set around the usable region. At least the usable region and the flat region of the substrate are electrically conductive and have a conductive portion. A plurality of first plated layers are formed in a periodical array pattern on the usable region as a microstructure array, and a radius distribution of the microstructure is within 5%. A second plated or electrodeposited layer is continuously formed on the conductive portion in the flat region and the first plated layers in the usable region.
These advantages and others will be more readily understood in connection with the following detailed description of the more preferred embodiments in conjunction with the drawings.